The invention relates to a method of cleaning sulphur and nitrogen containing flue gases by supplying gaseous reagent, preferably ammonia gas and converting the fumes with the reagent in a reaction chamber through which the reactants flow at a pressure of between 1 and 5 bar under the action of electron beams.
The method is principally provided for use in large boiler plants in order to minimize the emission of hazardous acid components of sulphur (SO.sub.x) and nitrogen (NO.sub.x) and thus to meet environmental requirements.
From a publication by the firm Ebara/Japan "Electron Beam Flue Gas Cleanup Process" a method is already known in which an approximately stoichiometric proportion of ammonia gas (NH.sub.3) is added to flue gas before the mixture is subjected to the action of electron beams. By this means the harmful components of the flue gas are converted to ammonium sulphate or ammonium nitrate, both of which serve as fertilizers.
A problem with the process described in this publication lies n introducing the electron beams into the reaction chamber. The sources of electron beams, in general electron emitting cathodes heated to a high temperature, can only operate in a high vacuum. The contents of the reaction chamber, however, are at atmospheric pressure or higher. Electron beams have the property of spreading in a gaseous atmosphere from collisions with the gas molecules, so that their range is markedly reduced after they emerge from the vacuum into atmospheric pressure. This is one reason why many electron beam treatment methods are carried out under high vacuum. A further problem encountered in the prior art involves creating the most homogeneous interaction between the reaction mixture on the one hand and the electron beams on the other.
In known methods electron beams are produced with an accelerating potential of 300 KV and injected into the reaction chamber through a "window" made out of a thin metal foil such as titanium/palladium. The window-foil acts as a separator wall between the high vacuum electron beam source and the atmosphere in the reaction chamber, but when sufficiently thin it is at least partially penetrable to the electron beams. The window-foil is not damaged by the high energy density of the electron beam by virtue of the latter being deflected periodically over the window surface by a deflection system associated with the beam source.
However, this prior art method and the apparatus required for it exhibit numerous disadvantages: the high acceleration potential of 300 KV required for penetrating the film window requires an expensively constructed electron gun. Also, the high accelerating potential is accompanied by a correspondingly strong X-radiation at the emission point, so that comprehensive radiation shielding is necessary. Moreover, the foil-windows result in a high loss of efficiency which with the necessary high electrical power leads to correspondingly high energy losses and also to an extraordinarily high thermal strain on the window. As a result of these factors, as well as through the interaction of the foil window with the reaction chamber contents, it is necessary to replace the window frequently, with a resulting relatively high proportion of downtime for the whole cleaning plant. This means that for continuous use of the boiler plant a number of cleaning plants must always be provided to run in parallel.
The present invention is thus in part directed to providing a process for cleaning flue gases through the reaction of the gases with ammonia gas under the action of electron beams, but with a higher operational safety level, a lower radiation level, and a higher efficiency than is found in the prior art.